The "perched water table" soil profile design as currently employed in U.S. Golf Association specifications on putting greens, some natural turf athletic fields, and some turf race courses consists of a sand-based rootzone soil mix underlain by a gravel drainage blanket, and a thin, coarse sand interface between the rootzone and the gravel layer which is termed the "choker layer". One or more drain pipes extend in trench(es) formed in the gravel drainage blanket to collect water and direct the water to a remote drain, creek or pond. An example of such a soil profile is illustrated in FIG. 1.
In this soil profile design, a "perched water table" (also referred to as a "capillary fringe") is created in the lower portion of the rootzone. The "perched water table" is the zone of saturated or nearly saturated soil in the lower portion of the rootzone due to the large particle size differences between the rootzone mix components and the choker layer. The formation of a "perched water table" in this soil profile represents an equilibrium hydrologic condition that occurs at some time after excess rainfall or irrigation water has drained from the soil profile. In this equilibrium condition, the gravitational forces attempting to drain water from the rootzone are balanced by capillary forces retaining water in the rootzone mix. This equilibrium condition is illustrated by a capillary tube model shown in FIG. 2. The purpose in creating this "perched water table" is to provide a reservoir of water in the lower layers of the soil profile for subsequent uptake by the turf growing on the soil surface.
In typical applications, the rootzone soil mix will extend downwardly from the turf layer about 12-14 inches (31-36 cm), the sand interface will extend downwardly from the rootzone about 1-2 inches (2.5-5.0 cm), and the drainage blanket will extend downwardly a further 4-5 inches (10.25-13.00 cm). The width of the capillary fringe is equal to the capillary rise from the textural difference interface into the rootzone. The degree of soil saturation as a function of elevation above the choker layer is illustrated in FIG. 3.
The extent of capillary rise in the soil is dictated by the pore size distribution of the rootzone mix. The pore size distribution is primarily influenced by the size of the sand and/or soil particles making up the mix. Generally, a rootzone mix composed of relatively smaller particles creates a higher capillary rise, and vice versa (see, e.g., Table 1).
TABLE 1 ______________________________________ Height of Capillary Rise for Different Soil Materials (From Bear, 1972) Material Capillary Rise (cm) ______________________________________ Coarse Sand 2-5 Sand 12-35 Fine Sand 35-70 Silt 70-150 Clay &gt;200-400 ______________________________________
However, there can be problems associated with this "perched water table" soil profile design in its current configuration. In particular, if the soil or sand particles in the rootzone mix are too small, the water will perch to an excessively high level in the profile (i.e., the capillary rise will be too large) leading to waterlogging of the roots and turf decline. On the other hand, if the sand or soil particles are too large, the water will perch low in the soil profile (i.e., the capillary rise will be too small) leading to droughty soil conditions requiring more frequent irrigation.
Moreover, often the concern is not so much with the height of capillary rise in the soil profile but rather with the rate of downward water movement through the soil profile. Consider the situation where a heavy rainfall occurs during a football game. To maintain the field in good playing conditions (i.e., not muddy or slippery) the rainfall must quickly infiltrate into the soil. Also, consider the situation of a steady rainfall during PGA tournament play. To avoid the costly cancellation of the round, the rainfall must quickly infiltrate into the soil on the putting greens.
Some systems have been developed to remove excess water from a soil profile. For example, Daniel, et al., U.S. Pat. No. 3,908,385, issued Sep. 30, 1975, shows a subsurface multistrata base and drainage pipe combination designed for vacuum pumping or irrigation. The Daniel system applies a vacuum to the drain pipes to strip rainfall down through the turf and through a uniform porous media between the compacted subsoil and the final grade level. The pipes collect the water and carry the water to an outlet drain. However, vacuum pumping can have drawbacks, namely, a complex (and hence expensive) array of pipes and vacuum equipment to remove the water from the soil.
Another system for removing water is shown in Imbertson, et al., U.S. Pat. No. 2,127,175, issued Aug. 16, 1938. In one embodiment, Imbertson shows a pipe extending vertically downward through the soil wherein the deepest end of the pipe has a perforated portion surrounded by pea gravel. A vacuum is applied to the pipe and air is removed from the voids or spaces between the soil particles, thus drawing water into the soil by gravity and pressure differential. It is believed that the pea gravel surrounding the pipe acts to prevent the soil from clogging the perforations in the pipe during use.
In another embodiment, Imbertson shows perforated pipes extending horizontally through the soil to draw a vacuum below the turf layer. The horizontal pipes remove the air from under the saturated lawn to cause the water to penetrate the soil.
However, the Imbertson system can also have certain drawbacks. In particular, it is believed that the Imbertson air pipe system does not uniformly control the air pressure under a lawn because of the relatively small spaces or voids within the soil which restrict air movement. There can therefore be a large pressure differential only a relatively short distance from each air pipe. Therefore, to provide a sufficient vacuum across the entire lawn can require: (i) a multiplicity of air pipes; and/or (ii) equipment capable of maintaining a strong vacuum. As indicated previously, such systems can be expensive to operate and maintain.
In any case, none of the systems described above provide a simple method and apparatus for the uniform control of air pressure under the turf and for adjusting the equilibrium condition between the capillary rise and the gravitational forces on the water in the soil.